The Chemistry of Catalytic Hydrocarbon Conversions PDF
Preview The Chemistry of Catalytic Hydrocarbon Conversions
The Chemistry of Catalytic Hydrocarbon Conversions HERMAN PINES Department of Chemistry Northwestern University Evans ton, Illinois 1981 ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers New York London Toronto Sydney San Francisco COPYRIGHT © 1981, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. Ill Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 7DX Library of Congress Cataloging in Publication Data Pines, Herman, Date. The chemistry of catalytic hydrocarbon conversions. Includes bibliographies and index. 1. Catalysis. 2. Hydrocarbons. I. Title. Q0505.P55 547.1'395 80-1778 ISBN 0-12-557160-7 PRINTED IN THE UNITED STATES OF AMERICA 81 82 83 84 9 8 7 6 5 4 3 2 1 To Dorothy, Judy, Jeff, David, and Debbie Preface The catalytic conversion of hydrocarbons is the key method for the production of both high octane gasoline and petrochemicals. A thorough understanding of the chemistry of reactions accompanying these conver- sions is of the utmost importance for developing new catalytic processes and for improving existing ones. Standard textbooks of organic chemistry pay only scant attention to this conceptually and industrially important field of chemistry. This book, with few exceptions, is confined to a description of the var- ious aspects of catalytic conversions of hydrocarbons. However, in view of the recent interest in synthetic fuels, it contains a description of the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen and from methanol. The material is arranged according to the type of catalyst used in the various reactions. The first chapter covers the acid-catalyzed reactions of hydrocarbons. Acid catalysts are widely applied in a multitude of catalytic reactions and are the backbone of many major petrochemical processes. Because of its length and vastness of material, this chapter is divided into eight sections, and the references are also grouped by these same section divisions. In the remaining seven chapters references are not separated. References with an asterisk contain either broad citations of pertinent literature or a review article. In writing the book an attempt was made to present mechanistic inter- pretations of the reactions, and, whenever possible, examples of indus- trial relevance were included. The author wishes to express his gratitude to Professor Charles D. Hurd who read the entire manuscript. His many valuable suggestions are greatly appreciated. The various chapters of the manuscript were ex- amined by experts in their respective fields. Drs. Fred Basolo, R. L. xi xii Preface Burwell, Jr., W. O. Haag, G. W. Keulks, Gordon Langlois, Jr., D. A. McCaulay, D. F. Shriver, Samuel Siegel, W. H. M. Sachtler, and W. M. Stalick. Their critical comments and constructive suggestions are gratefully acknowledged. This book would probably not have been written but for the urging and nudging from Professors John B. Butt and W. Keith Hall. I thank them now for it. Ms. Vonita Curbow deserves special recognition for the difficult task of typing the manuscript from the handwritten copy and for making india ink drawings of some of the chemical structures. All this she has per- formed skillfully and cheerfully. It is the author's hope that this book will be of value not only to all those active in the field of catalytic hydrocarbon conversions but that it will also be used as an auxiliary text for seniors and graduate students in organic chemistry and related areas. Herman Pines Terminology and Abbreviations For general abbreviations and terminology of catalytic systems consult 4'Manual of Symbols and Terminology for Physicochemical Quantities and Units—Appendix II. Part II. Heterogeneous Catalysis," Adv. Catal. 26, 351-392 (1977). Symbols used in this volume are listed below. Butane the unbranched C alkane 4 Butanes both butane and isobutane Pentane the unbranched C alkane 5 Pentanes includes the unbranched C alkane, and its isomers iso- 5 pentane and neopentane Ethene ethylene Propene propylene Butènes 1- or 2-butene. The term demands a chain length of four carbons, hence excludes the isomeric methylpropene (isobutylene) with chain length of three carbons Butylènes includes both straight chain and isobutylene Hexenes unbranched, acyclic C H 6 12 Propyl, butyl the radicals CH CH CH —, CH CH CH CH — 3 2 2 3 2 2 2 Butylbenzene applies to C H CH CH CH CH ; the prefix Az-butylben- 6 5 2 2 2 3 zene should be avoided GHSV gaseous hourly space velocity, e.g., volume of gas/ volume of catalyst/hour LHSV liquid hourly space velocity, e.g., volume of liquid/ volume of catalyst/hour WHSV weight hourly space velocity, e.g., weight of substrate/ volume of catalyst/hour Temperature expressed in °C, unless otherwise indicated xiii 1 Acid-Catalyzed Reactions I. INTRODUCTION Acid-catalyzed reactions are by far the most important reactions in- volved in the rearrangement and conversion of hydrocarbons. These reac- tions are responsible for isomerization of alkanes and cycloalkanes, po- lymerization of alkenes, catalytic cracking, alkylation of isobutane to high-octane hydrocarbons, reforming of naphthas, synthesis of alkylaro- matic hydrocarbons, etc. The relevant acids include three types: (a) pro- tonic acids, of which the commercially most important are silicophos- phoric acid, sulfuric acid, and hydrogen fluoride; (b) Lewis acids such as aluminum chloride and boron trifluoride; (c) acidic oxides represented by silica-alumina (cracking catalyst), zeolites, and "acidic" alumina, which contains small amounts of halides. It is generally agreed that acid-catalyzed hydrocarbon conversion reac- tions proceed by way of highly reactive, positively charged intermediates that are referred to variously as carbonium ions, cations, or carbenium ions. Since these intermediates appear prominently in the pages that fol- low, it will be helpful to consider their nomenclature, and the tertiary butyl case will be illustrative. 1. The tert-butyl group may be neutral, (CH ) C-, or it may carry a pos- 3 3 itive charge, (CH ) C+, or a negative charge, (CH ) C". These are named, 3 3 3 3 respectively, ter/-butyl radical, tert-butyl carbonium ion, and tert-butyl carbanion. The three situations are depicted by the separate words (not suffixes) radical, carbonium ion, and carbanion. Thus, (CH ) C+ is tert- 3 3 butyl carbonium ion (not teri-butylcarbonium ion); it should not be named either trimethyl carbonium ion or trimethylcarbonium ion. 2. /m-Butyl cation is also good usage. 3. The rationale underlying "carbenium" is simple. If a proton is added to carbene (methylene) the carbenium cation results, H C: 4- H+ —* 2 CH +, which is comparable to the formation of ammonium ion by adding a 3 ι 2 1 Acid-Catalyzed Reactions proton to ammonia. Thus, carbenium cation turns out to be nothing but methyl cation. Substitution of the three FTs by three methyls results in the name trimethylcarbenium cation. Note that "carbenium" in such terms must be a suffix, not a separate word. Since it offers no advantages and may be troublesome to some, "car- benium" is not used in this chapter. It should be kept in mind that an anion must be associated with a cation. In the case of (CH ) C+, if the 3 3 anion were HS0 ", the full name would be tert-butyl hydrogen sulfate. 4 A comment on the formation, reaction, stability, and geometry of car-- bonium ions will serve to simplify later discussions of the mechanism of acid-catalyzed reactions. A. Carbonium Ions Carbonium ions are produced most commonly in four ways. 1. Addition of the proton of an acid to an olefin: HX + C=C H—C—C The reaction is an acid-base reaction, and therefore the concentration of carbonium ion depends on the strength of the acid. 2. Removal of a halide ion from an aliphatic halide by a Lewis acid, as in the action of aluminum chloride on an alkyl halide: R—Cl + A1C1 —> R+ + AlCLr 3 3. Oxidation of tertiary alkanes. Oxidation of isobutane by sulfuric acid is an example: ch ch 3 3 + CH3CIH 4H9SOa *-CH,CI + 2H,0 + SOa + 3 HSO/ CH3 CH 3 4. Addition of acids to alcohols. The initial step involves the formation of a relatively stable oxonium salt, which decomposes under suitable con- ditions: -ι + Η HX R—Ο—H I + Χ Η 0 Ο-Η Χ ^= 9 Owing to their electronical-deficient nature, carbonium ions undergo a variety of reactions. These reactions, which form the basis of isomeriza- tion mechanisms, also explain how the side reactions accompanying many isomerizations come about. I. Introduction 3 1. Loss of a proton from an adjacent carbon atom: H + -H + HC—C—C—CH • CHXH=CHCH + H 3 3 3 II H H 2. Internal rearrangement by a hydride ion shift: Hl \+ + H I -H: HC—C—Ç—CH •H C-Ç— Ç— CH 3 3 3 3 HC H HC H 3 3 Such shifts occur rapidly and are a consequence of the difference in the stability of carbonium ions. The order of increasing stability of carbonium ions is primary < secondary < tertiary and alkyl < allyl. 3. Internal rearrangement by migration of R: to an adjacent carbon: HC H1 + H 3 I ~CH : + I 3 HC-—C—C—CH HC Ç C CH 3 3 3 3 HC HC CH 3 3 3 4. Removal of a hydride from another molecule. (a) In the presence of 98% sulfuric acid, the intermolecular hydride re- moval occurs only between a tertiary cation and molecules with hydrogen bonded to tertiary carbons: + (CH ) C + (CH ) CHCH CH • (CH ) CH + (CH ) CCH CH 3 3 3 2 2 3 33 32 2 3 (b) Aluminum chloride/hydrogen chloride also catalyzes intermolecu- lar transfer of hydride ion from a secondary carbon atom to a secondary or tertiary cation: (CH)C+ + CH CH CH CH ?=± (CH ) CH + CH CH CHCH 33 3 2 2 3 33 3 2 3 (c) The removal of hydride from a primary carbon atom to a cation occurs intermolecularly only in the presence of very strong acids (super- acids), such as FS0 H/SbF : 3 5 (CH)CH+ + (CH )C • (CH ) CH + (CH)CCH+ 32 34 3 2 2 33 2 (d) Owing to the fact that an allylic cation is more stable than the corre- sponding alkyl cation, 96% sulfuric acid is able to catalyze the transfer of a secondary allylic hydrogen to a cation: + 1 1 (CH ) C + RCH=CHCH R • (CH ) CH + RCH=CHCÏiR 3 3 2 33 R = alkyl 5. Addition of a carbonium ion to an olefin or to an arene: + (a) (CH )C + CH=:C(CH) (CH)CCHC(CH) 33 2 32 33 2 32 4 1 Acid-Catalyzed Reactions ,c„,,,c. Q > ς ^„ M 6. ß-Scission of a carbonium ion to another carbonium ion and an ole- fin: HC CH 3 3 1 1+ + H C-C-CHyC—CH *-(CH ) C + CHCH=C(CH) 3 3 33 3 32 HC ^Η 3 3 This reaction occurs most readily with ions capable of generating a ter- tiary cation on scission. It is to be noted that this is the reverse of reaction 5(a). 7. Rapid exchange of α-hydrogens of cations with an acid. Exchange with sulfuric acid-d is an example: 2 + + 9D S0 + (CH ) C • (CD) C + 9DHS0 2 4 3 3 3 4 The carbonium ion is planar, and the valence bond angles of the posi- tively charged carbon are 120°. This fact has proved extremely useful in yj 120 the study of carbonium ion mechanisms. If an asymmetric carbon is con- verted to a carbonium ion, the asymmetry is lost because of the planar character of the ion. Thus, racemization of an optically active hydrocar- bon during a reaction proceeding by a carbonium ion mechanism indicates that the asymmetric carbon bore a positive charge sometime during the reaction. Also, the rate of racemization often gives valuable information concerning the mechanism of reaction. The anions associated with cations R + are not shown above and usually are omitted throughout the text. The omission is for the sake of conve- TABLE 1.1 0 Heat of Formation of Alkyl Cations Alkyl cation Δ// (kcal/mol) 2β9 CH+ 261 3 + CH CH 219 3 2 + CH CH CH 208 3 2 2 (CH)CH+ 192 32 + CH CH CH CH 201 3 2 2 2 (CH)CHCH+ 199 32 2 CH CH CHCH 183 3 2 3 + (CH ) C 167 3 3 a From Lossing and Semeluk (1970).
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